Knight, JT 2008, 'Aspects of the biology and conservation of the endangered Oxleyan pygmy perch Nannoperca oxleyana Whitley', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright JT Knight 2008
The endangered Oxleyan pygmy perch Nannoperca oxleyana Whitley is a small-bodied freshwater percichthyid fish endemic to coastal mid-eastern Australia. In this study, several poorly understood aspects of the species’ biology were described and related to the ways that environmental variables and anthropogenic impacts may have influenced current patterns of distribution and abundance. Using this information, a number of recovery-based management and research principles for the conservation of the species were developed.
Initially, a standardised, non-destructive sampling protocol was designed that effectively and efficiently detected the presence, and quantified the relative abundance, of extant populations of N. oxleyana. Through evaluation of field experiments and the analysis of survey data a protocol was recommended that includes saturating sites with unbaited traps set for at least 30 minutes and sampling with a backpack electrofisher. It was determined that seine netting should be reserved for situations where an electrofisher is unavailable or non-deployable.
Using this protocol, the distribution and habitat associations of N. oxleyana in south-eastern Queensland and north-eastern New South Wales (NSW) were documented. The species’ range encompasses approximately 530 km of coastline from Coongul Creek on Fraser Island, Queensland (25º 16’S, 153º 09’E) south to Tick Gate Swamp near the township of Wooli, NSW (29º 54’S, 153º 15’E). It is confined primarily to dystrophic, acidic, freshwater systems draining through sandy coastal lowlands and Banksia-dominated heath ecosystems. Both lentic and lotic environments provide habitat for the species but it is found only in slow flowing pools and backwaters of river channels and tributaries as well as in swampy drainages, lakes, ponds and dams. Beds of emergent or submerged plants, steep/undercut banks fringed with the semi-submerged riparian vegetation, leaf litter and snags were defining microhabitat features. Recent and historical survey data suggest that human activities have had a significant influence on contemporary species presence/absence patterns and may have been responsible for the prominent fragmentation within its distribution.
Mitochondrial DNA (mtDNA) control region variation was used to assess genetic diversity and structure across the geographic range of N. oxleyana. Haplotypic diversity was highest in a small NSW subcatchment south of Evans Head (h = 0.594) followed by in Marcus Creek in Queensland (h= 0.475). Distinct genetic differentiation was evident among Queensland localities and NSW subcatchments, implying restricted gene flow between coastal river systems. One of the nine haplotypes detected was distributed over 83.4% of the species’ range, suggesting historical connectivity among the now fragmented populations. These patterns were concordant with eustatic changes associated with the last glacial maximum. High barrier sand dunes may also act as barriers to gene flow and dispersal between adjacent NSW subcatchments. Conservation efforts should focus on the preservation of genetic diversity by maintaining as many genetically differentiated populations as possible. The relatively diverse populations inhabiting the South Evans Head subcatchment and Marcus Creek require special management consideration.
The reproductive biology of N. oxleyana was described from simultaneous studies of wild populations in north-eastern New South Wales and mature fish held in aquaria. In the wild, males and females matured at total lengths of 24.0-25.9 mm and 28.0-29.9 mm, respectively. In captivity, male broodfish closely guarded sites within artificial, plant-like substrate in which pairs of fish spawned adhesive eggs. Protracted serial spawning of wild and captive fish occurred from September to April/May at mean water temperatures ≥ 16.6° C and day length ≥ 10.7 hours. Captive broodfish spawned on an average of 57% of days during the 256 day spawning period. Mean relative and batch fecundities of captive females were 587 eggs/g of body weight and 7.8 eggs/fish/day, respectively. Batch fecundity of wild females was estimated at 7.8 eggs/fish. The protracted serial spawning strategy of the species may reflect an evolutionary adaptation for survival in the harsh, variable environments in which it occurs.
The developmental ontogeny and morphology of the eggs, larvae and early juveniles were described based on collections of preserved wild fish, and preserved and live captive specimens reared at 25±1º C. Eggs are telolecithal, spherical, average 1.02±0.004 mm in diameter, have a smooth chorion without filaments that adheres to spawning substrate, and follow the general pattern of teleost embryogenesis. Early, middle and late stages of embryonic development were completed on average at 16, 28 and 50 hours post fertilisation. The larvae have generalised perciform morphological development with no apparent larval specialisations. Live newly hatched larvae measured 2.8-3.4 mm in body length and commenced exogenous feeding at five days post hatch. Squamation occurred in larvae from 7.5-10.3 mm (preserved body length) with its completion determining the end of the larval stage. Given that N. oxleyana utilise aquatic vegetation throughout its entire life cycle, the conservation and recovery of the species depends largely on the maintenance of this habitat within remaining water bodies and associated wallum ecosystems.
The results of this study have important implications for the conservation of N. oxleyana in Australia. It is evident that the recovery of this species is dependent upon the protection of the particular macro-, meso- and microhabitat features shown to be associated with healthy extant populations. This will require management strategies focused on maintaining environmental drivers such as natural climatic and river flow regimes, riparian vegetation and nutrient dynamics in these dystrophic ecosystems. In addition, it will be necessary to understand the processes that govern the dispersal of this small fish along connectivity pathways in floodplain rivers, as well as identifying the main breeding populations, sources of colonists and possible movement pathways into individual drainages and isolated water bodies. In this regard, population genetics research using sensitive molecular techniques such as microsatellite markers may elucidate patterns of contemporary gene flow and dispersal among drainages as well as inform efforts to increase within-population genetic diversity of remnant populations suffering the effects of small population size. This thesis has laid the foundation for these research initiatives.